# CVE-2020-6840: Use after free in n/a

> A use-after-free vulnerability in mruby's hash_slice function allows attackers to potentially execute arbitrary code or cause a denial of service.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 11 Jan 2020
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-4v2f-5xhv-8ff4, UBUNTU-CVE-2020-6840

## What is CVE-2020-6840?

CVE-2020-6840 is a critical use after free vulnerability in n/a, published on 11 Jan 2020. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

In mruby 2.1.0, there is a use-after-free in hash_slice in mrbgems/mruby-hash-ext/src/hash-ext.c.

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2020-6840?

It affects 1 product.


Products: Mruby Mruby.

## How do I fix CVE-2020-6840?

No fix has been published for CVE-2020-6840 yet. Limit exposure of the affected software until one is available.

## Is CVE-2020-6840 being exploited?

There is no known exploitation of CVE-2020-6840 in the wild and no public exploit code.

## Other vulnerabilities in n/a

- [CVE-2020-6838](https://insights.sbomify.com/CVE-2020-6838/): Use after free in n/a (critical 9.8)
- [CVE-2020-6839](https://insights.sbomify.com/CVE-2020-6839/): Memory corruption in n/a (critical 9.8)
- [CVE-2018-10199](https://insights.sbomify.com/CVE-2018-10199/): Use after free in n/a (critical 9.8)
- [CVE-2022-1212](https://insights.sbomify.com/CVE-2022-1212/): Use-After-Free in str_escape in mruby/mruby in mruby/mruby (critical 9.3)
- [CVE-2021-4110](https://insights.sbomify.com/CVE-2021-4110/): NULL Pointer Dereference in mruby/mruby (critical 9.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2020-6840 and fix it if it is affected.

What it is: CVE-2020-6840 is a critical use after free vulnerability in n/a, published on 11 Jan 2020. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2020-6840 in the wild and no public exploit code.

Affected product: n/a. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2020-6840/ (Markdown: https://insights.sbomify.com/CVE-2020-6840/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2020-6840
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2020-6840
- OSV: https://osv.dev/vulnerability/CVE-2020-6840

---
From sbomify Insights: https://insights.sbomify.com/CVE-2020-6840/
Summaries are generated and may contain mistakes; the official description above is the record.